ISLAMABAD - At least 40 people were killed and over 135 others injured when twin blasts hit a residential area in Pakistan's southern port city of Karachi on Sunday night, local health official said.

Aftab Khatri, Health Secretary of Sindh said that the death toll may further rise 50 because the injured people including a large number of women and kids are in a critical condition.

Firefighters spray water to control a fire in a building after a bomb blast in a residential area in Karachi, March 3, 2013. [Photo/Agencies]

Senior intendant of Karachi Police Fayyaz Khan said that first bomb was triggered off by a remote controlled device and explosive materials were laden in a mini-truck parked near the entrance gate of a residential "Iqra City" area of the metropolis.

He said that the Bomb Disposal Squad is busy in investigating about the nature of second blast and according to initial reports it seems that the second explosion was carried out by a suicide bomber who blew himself up in the residential building.

Fayyaz said that an estimated 150 kg of explosives were used in the blast, adding that the bomb contained ball-bearing and lethal chemical material which caused huge fire after the explosion.

The blast leveled two multi-story buildings comprising of 200 flats to ground and several people are believed to be stranded under the rubbles.

Following the blast, several nearby vehicles, over 150 shops and a building caught fire.

The injured people were shifted to Abbasi Shaheed Hospital, Patel hospital and Jinnah Hospital.

The blast happened at about 7:00 pm (local time) at Abul Hassan Isphahani road of Karachi, the capital city of the country' s southern Sindh province.

No group claimed responsibility for the blast yet.

Both President Asif Ali Zardari and Prime Minister Raja Parvez Ashraf condemned the blast and directed hospital administration to provide best possible medical treatment to the injured people.

Governor Sindh also condemned the blast and announced a one-day- mourning in the province on Monday, saying that the national flag would fly at half-mast on all state-owned buildings across the province on the day of mourning.